An engine, which is an example of an internal combustion engine, has been reduced in size, and there are growing needs for an increased low-speed torque and improved responsiveness. An electric centrifugal compressor is attracting attention as an approach to meet such needs. There is a system including a multi-stage electric centrifugal compressor including such an electric compressor, with a low-pressure stage turbocharger disposed in an intake passage and an exhaust passage of an engine, and an electric compressor disposed in the intake passage downstream of a low-pressure stage compressor of the low-pressure stage turbocharger (see Patent Document 1).
This electric two-stage system is configured to perform supercharging in two stages: the first-stage supercharging by the low-pressure stage turbocharger, and the second-stage supercharging by the electric compressor.
The low-pressure stage turbocharger includes a low-pressure stage turbine disposed in the exhaust passage and driven by exhaust gas, and a low-pressure stage compressor disposed in the intake passage and driven by rotational torque of the low-pressure stage turbine. The electric compressor is disposed in the intake passage on the downstream side of the low-pressure stage compressor, and includes a high-pressure stage compressor driven by rotational torque of the electric motor.
In the above electric two-stage system, if the engine is operating in a range of low-speed rotation, the electric compressor is driven to increase the rotation speed of the high-pressure stage compressor to assist supercharging of intake air by the low-pressure stage compressor, which prevents delay in supercharging and enables improvement of low-speed torque and responsiveness.